1. Field of the Invention
The present invention relates to a sheet post-processing unit and an image forming apparatus, such as a copying machine, a laser printer, a facsimile and a multifunction machine of those machines, for carrying out post-processing jobs such as stacking, aligning and binding on sheets discharged from the image forming apparatus.
2. Description of Related Art
Hitherto, there have been known a sheet post-processing unit, and an image forming apparatus equipped therewith, for carrying out post-processing jobs such as a stacking job of stacking a plurality of discharged sheets, an aligning jobs of aligning the plurality of stacked sheets and a binding (stapling) job of binding the plurality of aligned sheets as disclosed in Japanese Patent Laid-Open Nos. 2002-274734 and 2002-37512 for example. As shown in FIG. 30, this unit binds a bundle of sheets after discharging the sheet by flying to a stapling tray 902 by means of sheet discharging rollers 901a and 901b, moving the sheet in the direction along the sheet conveying direction to align the both widthwise edges of the sheet by width aligning members 903a and 903b and others, and aligning the edges of the sheet in the conveying direction.
In order to do that, the sheet post-processing unit is equipped with an elastic member called a paddle 903 and an aligning belt 904 that rotates in synchronism with discharging rollers 901a and 901b for discharging sheets onto the stapling tray 902. The unit is arranged to align the sheet in the conveying direction by pulling back the sheet by the paddle 903 to a nip point between the aligning belt 904 and the stapling tray 902 and by hitting the sheet against a hitting alignment member 905 by frictional force of the aligning belt 904. It is noted that although the alignment belt 904 is shown to have a small diameter in FIG. 30 to simplify the drawing, it actually has a diameter larger than that shown in FIG. 30 and is disposed at the position closer to the stapling tray 902.
The sheet which has been discharged onto the stapling tray 902 and whose edge in the sheet conveying direction has been aligned by hitting against the hitting alignment member 905 is subjected to another operation of aligning the widthwise edges of the sheet which is carried out by sandwiching the sheet by width aligning members 903a and 903b in the direction orthogonal to the sheet conveying direction.
Accordingly, since the prior art sheet post-processing unit aligns the widthwise edge of the sheet by moving the width aligning members 903a and 903b movable in the sheet width direction as means for aligning the sheet in the width direction, it requires a wide stapling tray. Still more, assuming that the sheet extends/contracts due to temperature, humidity and others and that the sheet size subtly changes due to that, the unit is arranged so as to be able to absorb such changes by a certain degree by providing springs for example in the width aligning members 903a and 903b, it is unable to absorb the change exceeding the flexibility of the spring, thus possibly causing a buckling of the sheet and of causing a disturbance in the alignment.
Still more, in aligning the sheet in the conveying direction by hitting the edge of the sheet against the hitting alignment member 905 by the frictional force of the aligning belt 904, the prior art sheet post-processing unit has a possibility of causing a disturbance in the aligned bundle of sheets due to a subtle instability of the aligning belt 904, which occurs in rotating the aligning belt 904.
Further, since the prior art sheet post-processing unit utilizes the frictional force obtained in rotating the paddle as means for aligning the sheet in the sheet conveying direction, there is a possibility of causing buckling when the sheet hits against the hitting section because the deflection of the paddle increases with the increase of number of stacked sheets, thus increasing the contact pressure of the paddle against the sheet.
Still more, because the discharging rollers 901a and 901b discharge the sheet as if they kick (fly) out the sheet in discharging to the stapling tray 902, the sheet is not stably conveyed to the hitting section in such a case when the sheet is disorderly discharged or when a type of sheet that will not stably fall is conveyed.
Since the sheet discharged to the stapling tray 902 is just returned and stacked by the paddle 903 and the aligning belt 904, it is necessary to adjust the size and angle of installation of the stapling tray to prevent the sheet from being dragged by the succeeding sheet and to stably return the sheet.
The prior unit also requires many independent devices such as the mechanisms for aligning the sheet in the conveying and width directions as described above to carrying out the above-mentioned operations, thus increasing the complexity and size thereof.
It is therefore an object of the invention to provide a sheet post-processing unit and an image forming apparatus capable of stably conveying sheets and of lessening disturbances in aligning the sheets with a simple arrangement having a small processing tray that is less influenced by the angle thereof and others.